1. Field of the Invention:
This invention relates to the art of papermaking, and particularly to a method of treating starch-containing paper product at high temperature to improve its properties, including dry and wet stiffness and wet tensile strength.
2. Description of the Prior Art:
In the art of papermaking, it is customary to subject felted fibers to wet pressing and then to drying on heated rolls.
There is currently considerable interest in improving various properties of paper and boards. Quantifiable paper properties include: dry and wet tensile strength, folding endurance, stiffness, compressive strength, and opacity, among others. Which qualities should desirably be enhanced depends upon the intended application of the product. In the case of milk carton board, for example, stiffness is of utmost importance, whereas for linerboard three qualities of particular interest to us are strength, folding endurance, and high humidity compression strength.
All of these properties can be measured by well-known standard tests. As used herein, then, "wet strength" means wet tensile strength as measured by American Society for Testing and Materials (ASTM) Standard D829-48. "Folding endurance" is defined as the number of times a board can be folded in two directions without breaking, under conditions specified in Standard D2176-69. "Stiffness" is defined as flexural rigidity and is determined in a standard TAPPI test as the bending moment in g-cm at a fifteen degree deflection angle. "Linerboard", is a medium-weight paper product used as the facing material in corrugated carton construction. Kraft linerboard is linerboard made according to the kraft process, and is well known in the industry. Folding carton board is a medium to heavy weight paper product made of unbleached and/or bleached pulps having basis weights from 40-350 g/m.sup.2.
Prior workers in this field have recognized that high-temperature treatment of linerboard can improve its wet strength. See, for example E. Back, "Wet stiffness by heat treatment of the running web", Pulp & Paper Canada, vol. 77, No. 12, pp. 97-106 (December 1976). This increase has been attributed to the development and cross-linking of naturally occurring lignins and other polymers, which phenomenon may be sufficient to preserve product wet strength even where conventional synthetic resins or other binders are entirely omitted.
It is noteworthy that wet strength improvement by heat curing has previously been thought attainable only at the price of increased brittleness (i.e., reduced folding endurance). Embrittled board is not acceptable for many applications involving subsequent deformation, and therefore heat treatment alone, to develop the wet strength of linerboard and carton board, has not gained widespread acceptance. As Dr. Back has pointed out in the article cited above, "the heat treatment conditions must be selected to balance the desirable increase in wet stiffness against the simultaneous embrittlement in dry climates." Also, in U.S. Pat. No. 3,875,680, Dr. Back has disclosed a process for heat treating already manufactured corrugated board to set previously placed resins, wherein the specific purpose is to avoid running embrittled material through a corrugator.
It is plain that improved stiffness and wet strength, on one hand, and improved folding endurance, on the other, were previously thought to be incompatible results.
Every year, the paper industry consumes millions of pounds of starch--an inexpensive natural polymer closely related to cellulose in chemical composition. Preparations of starch are added to papers and board compositions principally to improve their dry strength and their surface properties (J. P. Casey, Pulp and Paper, 3rd edition, pp. 1475-1500, 1688-1694, 1981). However, despite the well-known uses of starch, and of heat treating, separately, papers containing starch have not previously been heat treated to improve wet strength. Indeed, one of ordinary skill would not have expected heat treatment to improve starched paper, since unlike protein, starch does not cross-link when heat is applied.
We have found that heat treatment unexpectedly improves the wet strength of papers and boards containing starch. In its broadest sense, the invention comprises steps of (1) adding starch preparation into the pulp slurry or onto surface of formed paper or board; and then (2) heating the said paper or board to an internal temperature of at least 400.degree. F. (205.degree. C.) for a period of time sufficient to increase the wet strength of the product.
This method produces a product having folding endurance greatly exceeding that of similar product whose stiffness and wet strength have been increased by heat alone, or by starch addition alone. This is clearly shown by the results of our tests, reported below.
If starch is added to the surface of a web, it may be in its native anionic form. However, when starch is added to an aqueous slurry, we prefer to render it cationic, and therefore more soluble, by pretreating it with quaternary ammonium ion salts to give the starch chains net positive charges. Such salts do not affect the paper strength.
We prefer to raise the internal temperature of the board to at least 450.degree. F. (232.degree. C.) during the heat treating step, as greater stiffness and wet strength are then achieved. This may be because at higher temperatures, shorter step duration is necessary to develop bonding, and there is consequently less time for fiber degradation to occur. Also, shorter durations enable one to achieve higher production speeds.
While the invention may be practiced over a range of temperatures, pressures and duration, these factors are interrelated. For example, the use of higher temperatures requires a heat treating step of shorter duration, and vice-versa. For example, at 550.degree. F. (289.degree. C.), a duration of 2 seconds has been found sufficient to obtain the desired improvements, while at 420.degree. F., considerably longer is required.
As an additional step, we prefer to rewet the product, immediately after the heat treatment, to at least 1% moisture by weight. These steps are followed by conventional drying and/or conditioning of the treated product. Of course, those skilled in the art will recognize the necessity of conditioning to a normal moisture content after treatment at high temperature. See, for example, U.S. Pat. No. 3,395,219. A certain amount of rewetting is normally done, and in fact product properties are never even tested prior to conditioning. All conventional rehumidification is done after the product has substantially cooled.
Our rewetting treatment principally differs from conditioning in that we add water, by spraying or otherwise, to a very hot and dry paper or board at the very end of the heat treatment, without intermediate cooling. It is important that water be applied to the product while it is still hot, certainly above 100.degree. C. (212.degree. F.), and preferably above 205.degree. C. (400.degree. F.). Another heat treatment or drying step may follow rewetting, on or off the machine, during a subsequent operation such as sizing, coating or calendering.